The invention relates to an apparatus for machining workpieces by stock removal, in particular the main and/or connecting rod bearing journals of crankshafts. Such apparatuses are known, for example, from the publication of the PCT application WO 96/17705 or the German patent specification DE 196 26 627 C1.
In the apparatus as known from DE 196 26 627 C1 the workpiece is held during the machining in a workpiece fixture which is equipped with at least one turntable drive. The turntable drive is used to rotate the workpiece, which is otherwise stationary, about its longitudinal axis. At least two milling cutters are provided in the known apparatus to machine the workpiece, which cutters machine the workpiece simultaneously. All advance motions which are required for example as a result of the eccentricity of the surface areas to be machined during the machining of the connecting rod bearing journals of crankshafts are performed by separate actuating drives of the milling cutters.
The advantage of the apparatus known from DE 196 26 627 C1 is that short machining cycles are achieved with highly specialized tool units in the apparatus per se. In this process, the machining operations are performed simultaneously by several milling cutters. The disadvantage is, however, that when two main and connecting rod bearing journals are machined simultaneously by means of two discoid milling cutters the cutters must be arranged with a left-hand and right-hand design. The simultaneous machining of two main or connecting rod bearing journals of a crankshaft additionally has an unfavourable effect on the precision of the machining.
An apparatus is known from WO 96/17705 in which the machining of the workpiece is also performed simultaneously by two discoid cutters. In contrast to the aforementioned known apparatus, the apparatus as described in WO 96/17705 comprises two workpiece fixtures and a conveying device for conveying the workpiece. The workpiece fixtures are flush with respect to one another and with respect to the conveying devices arranged in front of and behind the apparatus, so that the conveying device can convey the workpiece in a linear movement from the first conveying apparatus to the first workpiece fixture and from there to the second workpiece fixture and further to the second conveying apparatus. The discoid cutters are linearly movable between the two workpiece fixtures, so that they can first perform machinings on the workpiece gripped in the first workpiece fixture in a first working position and thereafter machinings on the workpiece gripped in the second workpiece fixture in a second working position.
For machining purposes the workpiece which in the known apparatus reaches the apparatus by way of the first conveying apparatus is loaded by the conveying device at first into the first workpiece fixture. Thereafter the workpiece is machined, with the discoid cutters assuming their first working position. After the end of the first machining operations the workpiece is removed by the conveying device from the first workpiece fixture and brought to the second workpiece fixture. Then further machining is performed by the discoid cutters on the workpiece gripped in the second workpiece fixture. During the machining steps the workpiece is turned by the turntable drive of the workpiece fixture. The advance and longitudinal motions are performed by the discoid cutters during the machining. Once the workpiece is gripped in the second workpiece fixture the conveying device can bring a new workpiece to the first workpiece fixture, so that the discoid cutters can commence with the machining of the new workpiece immediately after ending the machining of the workpiece gripped in the second workpiece fixture.
In the apparatus as known from WO 96/17705 the flow of material is improved in such a way that such an apparatus can be included in an automated production sequence. For this purpose, however, it is necessary to rechuck the workpiece at least once during the machining. This not only leads to the problem of the additional machining time required for the rechucking and conveying process, but also to a considerable increase in the complexity of the apparatuses required for controlling and monitoring the machining as well as for automated transport of the workpieces within the apparatus per se.
The disadvantage in this case too is that when two main or connecting rod bearing journals of a crankshaft are simultaneously machined by means of two discoid cutters it is necessary to provide them with a left-hand and right-hand design.
The simultaneous machining of two main and connecting rod bearing journals of a crankshaft and the rechucking of the crankshaft also have an unfavourable influence on the precision of the machining.
It is the object of the present invention, based on the aforementioned state of the art, to provide an apparatus of the kind mentioned above which allows a high machining precision.